(29) Travel. Travel costs are allowable for expenses for transportation, lodging, subsistence, and related items incurred by employees who are in travel status on official business incident to the program. Such costs may be charged on an actual basis, on a per diem or mileage basis in lieu of actual costs incurred, or on a combination of the two. The charges must be consistent with those normally allowed in like circumstances in nonfederally sponsored activities. The difference in cost between first-class air accommodations and less-than-first-class air accommodations is unallowable except when less-than-first-class air accommodations are not reasonably available. Notwithstanding the provisions of paragraphs C (7) and (10), travel costs of officials covered by those paragraphs, when specifically related to grant programs, are allowable with the prior approval of a grantor agency.